steel-core: remove unused dev-dependency on env_logger

Author: m4rch3n1ng

Cargo.lock

@@ -96,7 +96,6 @@ polling = "3.7.3"
 [dev-dependencies]
 proptest = "1.1.0"
 criterion = "0.5.1"
-env_logger = "0.10.0"
 
 [build-dependencies]
 steel-gen = { path = "../steel-gen", version = "0.2.0" }